1. Field of the Invention
The present invention relates to a semiconductor laser device which is used for an optical disk system, an optical communication system or the like.
2. Description of the Related Art
A semiconductor laser device, which is designed to emit a laser beam by electron transition in a compound semiconductor having p-n junctions, provides a low driving voltage as compared to a solid-state laser device or a gas laser device, while it is small-size and lightweight. Therefore, it has been extensively used as a laser beam source for an optical disk system, an optical communication system or the like.
For example, the semiconductor laser device described above has a construction as follows. That is, on a substrate composed of n-type (referred to as “n-” hereinafter) GaAs, there are provided, by turns, a lower clad layer composed of n-AlGaInP, an active layer having a multiple quantum well structure, a first upper clad layer composed of p-type (referred to as “p-” hereinafter) AlGaInP, an etching stopper layer (sometimes referred to as “ESL” hereinafter) with a multiple quantum well structure composed of p-AlGaInP and GaInP, a second upper clad layer composed of p-AlGaInP and having a stripe-form protrusion, a contact layer composed of p-GaAs, and a p-electrode (for example, see Japanese laid-open patent publication No. Hei 10-125995 (paragraph [0016] and FIG. 1)). The top surface of the second upper clad layer is covered with an insulative film except the portion corresponding to the stripe-form protrusion.
Meanwhile, because the conventional semiconductor laser device, for example described in the above-mentioned publication, includes the etching stopper layer with the multiple quantum well structure composed of p-AlGaInP and GaInP, luminous efficiency is lowered. According to knowledge of the present inventors, it is assumed that the luminous efficiency is lowered by about 7% in the event that the etching stopper layer is provided, as compared to the case in which no etching stopper layer is provided (see FIG. 10 discussed later).
This is because providing the etching stopper layer enables the light distribution, which should be concentrated to the active layer, to expand on the upper clad layer side (see FIG. 7 later discussed) and light absorption occurs in the contact layer and so on. However, absence of the etching stopper layer causes such a problem that the selective etching for forming the stripe-form protrusion is unable to be properly carried out on the second upper clad layer.
In general, light absorption at the contact layer composed of p-GaAs can be prevented or suppressed, if the thickness of the second upper clad layer composed of p-AlGaInP is increased to prevent the light distribution from reaching the contact layer. In an index guide laser provided with a light-guiding structure, a laser beam is confined due to the difference of refractive indexes between the second upper clad layer composed of p-AlGaInP with the stripe-form protrusion and the layers on both sides, at the second upper clad layer. If a clad layer composed of a material with high resistivity is used, the resistance of the stripe-form protrusion is increased and the output characteristics or temperature characteristics at the time of high output may be deteriorated.